The present invention relates to a magnetic sensor system, in particular for sensing the motion of elements moved in a linear or rotary manner.
It is known per se that sensors that are sensitive to magnetic fields are used in many applications where contactless detection of motion is desired. The motion can be rotary or linear. A distinction must be made here between two fundamentally different measurement principles. In one case, one or more magnetic dipoles are mounted, as the active elements, on the element to be detected, and the motion is determined directly via the magnetic field, which changes with respect to time, at the point where the sensor is located. In contrast, with passive transmitter elements, which are made of a soft-magnetic material, the magnetic field is produced by a working magnet that is permanently connected with the sensor. The sensor measures the change in the magnetic field of the working magnet caused by the motion of the transmitter elements.
In addition to Hall technology, known per se, for measuring magnetic fields, “XMR” technologies, i.e., magnetoresistive measurement principles, are also finding increasing use, as an alternative, with passive transmitter elements in automotive applications. It should be noted that, unlike Hall sensors, XMR sensors detect the “in-plane” component of the magnetic field in the sensor element. For this purpose, previously common XMR sensors use a working magnet, the field of which must be adjusted such that the offset at the location of the sensitive element is zero, or a “backbias” field will be produced that defines the working point of the sensor.
For example, a concept is described in DE 101 28 135 A1 with which a hard-magnetic layer is deposited in the vicinity of, i.e., in particular on and/or under, a magnetoresistive layer stack. This hard-magnetic layer is then coupled—primarily via its stray field—with the magnetoresistive layers, thereby producing a “bias” magnetic field which acts as the magnetic field offset, so that, even when an external magnetic field superimposed on the internal magnetic field is varied even slightly, an easily measured and relatively great change in the actual measured value is obtainable, the measured value being detected as a change in resistance in the layer stack.
The sensors described above are often utilized in a “gradiometer” system in a manner known per se to measure rotational speed. This means that two branches of a Wheatstone bridge are separated by a specified distance, so that a homogenous magnetic field does not induce a bridge signal. In contrast, a variation of the magnetic field in the region of the predetermined distance produces a bridge signal. The sensor therefore only measures the signal from a magnetic rotor, the distance between the pairs of poles of which approximately corresponds to the predetermined gradiometer separation.
Unlike the use of XMR elements, which perform absolute measurements, the application of the gradiometer principle in a magnetoresistive XMR measurement bridge results in a reduction of the sensitivity of the sensors to homogeneous interference fields. An alignment of the magnets used previously, which was carried out so that the offset can be eliminated at both locations of the sensor elements of the gradiometer system, cannot be carried out in this case, however; although electronic alignment is possible, in principle, a relatively small signal is obtained here with a large offset.